The International System of Units, abbreviated SI (for the French phrase Système International d'Unités), is the most widely used system of units. Along with the older cgs (centimetre, gram, second) system, SI is sometimes
referred to as the metric system (especially in the United States, which has not widely adopted its use in everyday commerce, and the UK where conversion is incomplete).

SI is built on seven SI base units, such as the kilogram and metre. These are used to define various SI derived units. SI also defines a number of SI prefixes to be used with the units: these combine with any unit name to give subdivisions and multiples. For example, the prefix kilo denotes a multiple of a thousand, so the kilometre is 1,000 metres, the kilogram 1,000 grams and so on.

Symbols are written in lower case except for in symbols where the unit is eponymous, or derived from the name of a person. This means that the symbol for the SI unit for pressure, named for Blaise Pascal, is Pa, whereas the unit itself is written pascal. The official SI brochure lists the symbol for the litre as an allowed exception to the capitalization rules: either capital or lowercase L is acceptable.

Symbols are written in singular e.g 25 kg (not "25 kgs")

It is preferable to keep the symbol in upright roman type (for example, kg for kilograms, m for meters), so as to differentiate from (mathematical and physical) variables (for example, m for mass, l for length).

A space between the numbers and the symbols: 2.21 kg, 7.3·102 m2

SI uses spaces to separate decimal digits in sets of three. e.g. 1 000 000 or 342 142 (in contrast to the commas or dots used in other systems e.g. 1,000,000 or 1.000.000).

SI uses a comma to separate integers to decimal fractions. The number "twenty four and fifty one hundredths" would be written as 24,51 (in contrast to other systems where points are used, e.g. 24.51)

Americans frequently spell 'metre' as 'meter', and 'litre' as 'liter'; however 'metre' and 'litre' are the official BIPM names for these units, although the American usage has been approved by the US government. The official US spelling for 'deca' is 'deka', though Americans use the international spelling more often than the American one.

The unit 'gram' is also sometimes spelled 'gramme' in English speaking countries, though that is an older spelling. Several other languages use the American spelling. In written practice only the abbreviated (prefixed) symbols are used, qualifying the spelling issue.

The English language version of the official SI brochure (http://www.bipm.org/pdf/si-brochure.pdf) published by the BIPM uses the decimal point instead of the comma to separate the integer portion of a number from the fractional portion. This indicates that the use of the comma is not an inherent part of SI, but a part of the language and culture in which it is used in a particular case. In the U.S., the decimal point should be used.